This invention relates to quality assurance systems for surface treatment of a substrate surface of the type that measure the surface compressive layer depth or intensity of a surface treated substrate. such systems of this type generally provide the surface compressive layer depth measure directly and substantially without destructing the substrate. In particular, a substrate surface which has been surface treated, typically, by conventional shot peening techniques is evaluated by automated analysis of digitally recorded topographical line scans. The method involves computer analysis of the digitized line data to determine, typically, the diameter of the dimples generated by the shot impact. Since the depth of the plastically upset or compressive layer due to the shot peening roughly equals the dimple diameter, this method should substantially provide a direct measure of intensity. This invention relates to certain unique surface treatment analyzers and the evaluating means in association therewith.
During a shot peening process, a stream of shot (i.e., particles), travelling at a high velocity, is directed at a workpiece surface. The shot is directed at the workpiece so as to cause plastic deformation of the workpiece surface, which often is a metal surface. Although the process may be applied for other purposes, the shot peening process generally is used to increase fatigue strength of the workpiece.
For example, residual stress near the surface of high performance machine parts is directly related to the fatigue life of the part. If the surface is in a state of residual compression, the growth of microcracks created by, for example, hard processing, should be inhibited. Shot peening is, typically, a very effective means for producing surface compression residual stress, and therefore, prolonging the useful life of the part.
Determining the state of surface compression due to shot peening, however, has proven to be very difficult. There are currently several methods used to measure surface compression. In particular, there is a direct method for determining surface compression due to shot peening. Under this direct method the workpiece is cut by conventional cutting techniques, and then the depth, i.e. thickness, of the surface compression is physically measured. This direct method is time consuming and requires destructing the part being analyzed. A more advantageous system, then, would be presented if such amounts of time and destruction were reduced.
Another known method for determining surface compression due to shot peening which is less time consuming and avoids the destruction of the workpiece is referred to as an indirect Almen method. In the Almen method, a strip of material is shot peened, and then the strip is analyzed to determine the surface compression due to the shot peening. The Almen method is indirect in that the effects of shot peening are not measured directly from a workpiece, rather a substitute, namely, an Almen strip is utilized. However, the Almen strip method is subject to insensitivity due to process changes which may occur in the peening operations between Almen strip checks. Also, when peening workpieces having contoured surfaces, it is difficult to reproduce the peening conditions on the contour surfaces with an Almen strip which is usually flat. Further, variations in the Almen strips themselves render the Almen strip method subject to error. Consequently, a still more advantageous system would be presented if such amounts of insensitivity, inapplicability and variation could be reduced while still avoiding the destruction of the workpiece.
Finally, there has been developed a method and system for monitoring shot peening which utilizes two-dimensional, hereinafter referred to as 2-D, line trace information. Exemplary of such a prior art system is U.S. Pat. No. 5,003,805 issued Apr. 2, 1991 to Thompson entitled "A Method and System for Monitoring Shot Peening" and assigned to the same assignee as the present invention. While this system has met with a degree of commercial success, the system is limited in that only a mere 2-D view of the surface treatment intensity is presented. Therefore, a further advantageous system, then, would be presented if a more complete analysis of the surface treatment could be presented.
It is apparent from the above that there exists a need in the art for a substrate surface treatment quality assurance system which will not destruct the substrate in order to complete the evaluation, but which will evaluate the surface treatment done on the substrate surface in a manner which provides a full and complete analysis of the surface treatment through the use of an automated analysis of digitally recorded topographical lines. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.